The present invention relates to improvements to tuyere opening assemblies of vertical furnaces and reactors.
Vertical furnaces and reactors operating at high temperatures, such as, for example, blast furnaces and cupola furnaces, are conventionally heated by an assembly of burners or of hot-gas injection tuyeres arranged in a ring in relation to the interior of the furnace or reactor. The usually removable metal burners or tuyeres are seated in tuyere opening assemblies consisting of pieces of refractory material. These refractory pieces are subjected to high mechanical and thermal stresses and to considerable corrosion. A distinction can be made particularly between:
static mechanical stresses occurring as a result of the weight of the masonry and of the containment forces exerted by the outer metal casing of the furnace or reactor;
dynamic mechanical stresses occurring as a result of the abrasion brought about by the solid charge descending in the furnace or reactor and of the erosion caused by the streaming of molten glass, slag and/or metal along the pieces or by the hot gases rising in the furnace or reactor;
thermomechanical stresses attributable to the high temperatures prevailing in the furnace or reactor to the thermal gradients present in the stabilized operating mode or to the thermal shock occurring during the shutdown and restarting of the burners or the injection of hot gases through the tuyeres;
the corrosion which can take place either on the surface of the pieces in contact with the charge of the furnace or reactor or in the thickness of the pieces as a result of the infiltration or diffusion of aggressive elements emitted by the furnace or reactor charge.
These stresses and the corrosion experienced generate a process in which the pieces are destroyed.
Thus, for example, as regards tuyere opening assemblies blast furnaces, the component pieces of the assemblies are subject to cracking by thermal shock, to impregnation by potassium vapours condensing in the form of K.sub.2 O or K.sub.2 CO.sub.3 and attacking the refractory material in depth so as to make it brittle, and to wear by abrasion and erosion caused by the descending charge on the material embrittled by the previous attacks.
It is therefore essential that tuyere opening assemblies of vertical furnaces or reactors should have high resistance to thermal shock and high mechanical strength, which is maintained even in the event of a chemical attack in depth.
Tuyere opening assemblies are traditionally formed by assembling a large number of ceramised refractory keys, for example made of silicon carbide with a silicon nitride binder or based on corundum grains with a mullite or sialon binder, of different and relatively complex shapes.
This technology, although satisfactory, nevertheless involves a very high outlay because for the manufacturer it implies the production of a large number of moulds and the use of non-automated shaping techniques, such as manual ramming, and for the user it implies the need to provide qualified labour. Moreover, the assembly time on site is very long, typically approximately seven days for a ring of tuyeres of a blast furnace.
To save time during assembly on site, it has been proposed to supply sets of keys in preassembled form, that is to say, fixed together at the factory by known means, for example by the use of a cement. However, this technique does not afford any other advantage in comparison with traditional practice.
It has also been proposed to produce each tuyere belt or block no longer by assembling a large number of elementary keys, but by assembling a small number (for example, four) of suitably shaped elements obtained by moulding a refractory concrete. This procedure makes it possible to minimize the mould and labour costs and to save time during assembly on site: the assembly time can thus be reduced, for example, to 4-5 days instead of 7 days involved in the assembly of keys.
But experience has shown that, even with the use of the best available refractory concretes of high compactness, such as those described in French Patent 2,390,400 or the corresponding German Patent 2,759,809, the resulting tuyere opening assemblies exhibited, in use, a resistance to abrasion and to erosion under hot conditions far lower than that of high-temperature ceramised keys, particularly those of keys based on corundum grains and/or silicon carbide with a nitride or sialon binder, especially when there is simultaneously corrosion in the mass as a result of the infiltration or diffusion of aggressive elements, such as K.sub.2 O or K.sub.2 CO.sub.3.
Another version which has been tested involves producing the tuyere opening assemblies by assembling 4 carbon blocks machined to shape. This solution is unsatisfactory, however, because it leads to high thermal losses and the carbon blocks experience rapid wear by oxidation.
The object of the present invention is to provide improved tuyere opening assemblies for vertical furnaces or reactors which avoid the above-mentioned problems.